Innovative Engines: Thinking Outside the Box

Since the latter half of the 19th Century, the technology of choice to propel the motor car has been the reciprocating piston, spark-ignition Otto-cycle engine, followed by the compression-ignition diesel. After more than a century of service these engines have undergone development that has resulted in significant improvements in fuel efficiency, reduced emissions and improved outright performance.

However, the inherent inefficiencies of the reciprocating engine using the piston-connecting rod-crankshaft architecture have led innovative thinkers to experiment with alternatives that contradict mainstream wisdom. Not only are the mechanincal losses high, but the combustion process is also extremely inefficient.

A 19th Century design could be revived in the 21st century

In 1816, Scottish inventor Robert Stirling produced a closed-cycle engine where the working medium, in this case air, remains within the engine while the heat source for combustion is external to the engine.

Utilising pairs of pistons that operate together to provide a complete cycle, the air in one chamber is heated via heat transfer through the cylinder wall which forces the displacer piston, linked to a second power piston in the expansion chamber, to accelerate. The heated air continues to expand, displacing the power piston, which in turn drives a crankshaft. As the air cools, both pistons move back to their original positions, and the process repeats.

Image Credit: www.ridders.nu

Until recently, Stirling engines have mainly been used for stationary applications. However, the ability to run on alternative fuels has revived interest, especially as range-extender power units, where constant speed operation and low noise because of the external combustion are of interest.

Virtually all engines have either operated on a two- or four-stroke Otto cycle, where the entire combustion cycle takes place within any number of individual cylinders. During this cycle each cylinder undergoes intake, compression, power and exhaust phases. The idea of the split cycle, in which compression takes place in one cylinder while a second deals with power and exhaust, dates back to the late 19th century: Albeit with very little success.

100Kw per liter from a prototype Scuderi split cycle engine.

It took the Scuderi Group almost $65 million to develop a prototype that president, Sal Scuderi hopes will revive the fortunes of the split-cycle. Each engine module consists of two cylinders and pistons tied together through a crankshaft and high-pressure crossover passage. Because only air is being compressed in the first cylinder, a 75:1 compression ratio can be used. The outlet valve of the compression cylinder allows the high-pressure air to flow into the crossover passage where some cooling takes place.

Image Credit: www.autoevolution.com

As the inlet to the second cylinder opens, the high-pressure air charges the power cylinder where fuel is injected and ignited at about 15 degrees past top dead center. This timing is critical in ensuring that air is not recompressed, thereby significantly improving overall thermodynamic efficiency.

Scuderi claims a normally aspirated version of the engine is capable of producing up to 100Kw per liter, giving it better power density and lower fuel consumption than a conventional engine. An air-hybrid version using a high-pressure accumulator which is charged during vehicle coast-down could further improve efficiency by another 50 percent.

The Scuderi concept is compatible with spark-ignition operation on gasoline and alternative fuels or compression ignition with diesel fuel. The first functional Scuderi engine began testing on a dynamometer in mid-2009, and the company hopes to strike a production deal with an automaker in the near future.

With the industry focusing on weight reduction to meet the ever tightening emissions legislation, New Zealand’s Duke Axial Engine is lighter, more compact and already slightly more powerful than the typical equivalent conventional engine.

A revolutionary axial engine to lead the way?

Duke Engines has developed an axial engine prototype that completely does away with valves, while delivering excellent power and torque from a significantly smaller, lighter and simpler unit than current internal combustion engines.

Duke Engines' 3-liter, five cylinder test mule delivers 160Kw and 340Nm of torque at 4,500rpm; already an improvement over the two conventional 3 liter reference engines which are 20 percent heavier and take up three times the packaging volume. The innovative valveless ported design appears to be on track to deliver superior performance, higher compression and increased efficiency in an extremely compact and lightweight package with far fewer moving parts than conventional engines.

Being an axial design, the five cylinders encircle the drive shaft and run parallel to it, with the pistons driving a star-shaped reciprocator. This nutates around the drive shaft; similar to a spinning coin coming to rest on a table.

The reciprocator's center point drives the central drive shaft, which rotates in the opposite direction to the reciprocator. "That counter-rotation keeps it in tidy balance," says Duke co-founder John Garvey. "If you lay your hand on it while it's running, you can barely detect any motion at all; it's quite remarkable."

Image Credit: Duke Engines

With as many power strokes per revolution as a six cylinder engine, the axial engine delivers huge weight savings and a noteworthy reduction in the number of engine parts. Instead of mechanically- or pneumatically-operated intake and exhaust valves, the cylinders rotate past intake and outlet ports in a stationary head ring. During operation the cylinders slide past each port or spark plug, which are also mounted in the stationary ring, at the stage of the cycle it's required. By so doing, the Duke eliminates all the complexity of valve control and manages to run a five-cylinder engine with just three spark plugs and three fuel injectors.

The engine has shown excellent resistance to pre-ignition, partially because its cylinders tend to run cooler than comparable engines: Even with compression ratios as high as 14:1 running on regular 91-octane gasoline. This performance indicates that alternative fuels could hold further performance improvements. In a 2012 interview, Garvey said "we switched it over to kerosene jet fuel one day and it ran straight away; as well as, if not better, than it was running on petrol."

Having developed the engine to the point where it's ready to be commercialized, the small New Zealand company is still without funding. Even though the engine is suitable for a wide range of functions, the company needs a funding partner to develop it for a niche that can maximize the advantages.

Most car engines today are pretty similar. Even the ones we’d call different, like Porsche’s flat-sixes or Fiat’s two-cylinder, follow tried-and-tested engineering principles that have dominated the industry for the past 150 years. However, some of the nonconformist engines pose very interesting questions about existing designs and technology. Irrespective of the potential, is the industry likely to embrace any of these technologies after having invested more than a century and billions of dollars in development? Probably not!

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